1. Field of the Invention
The present invention relates to an organometallic complex which can convert an excited triplet state into light emission, a light emitting element using the organometallic complex, and a light emitting device using the light emitting element.
2. Description of the Related Art
A light emitting element using an organic compound is an element in which a layer containing an organic compound or an organic compound film emits light by applying an electrical field. A light emission mechanism of a light emitting element is as follows: electrons injected from a cathode and holes injected from an anode are recombined in the organic compound film to form a molecular exciton by applying a voltage to a pair of electrodes with the organic compound film interposed therebetween, and then energy is released to emit light when the molecular exciton returns to a ground state.
In such a light emitting element, generally, an organic compound film is formed by a thin film of less than 1 μm. In addition, since such a light emitting element is a self-light emitting element in which the organic compound film itself emits light, a backlight used for a conventional liquid crystal display is not required. Therefore, such a light emitting element has a great advantage of being able to be manufactured to be significantly thin and lightweight. In addition, for example, in a light emitting element having an organic compound film with a thickness of approximately 100 to 200 nm, the time from injection of carriers to recombination is approximately several tens of nanoseconds in consideration of the carrier mobility of the organic compound film, and the time required for light-emission is about microseconds or less, even when including a process from the recombination of carriers to the emission of light. Therefore, it is also one of features that the response speed is quite fast. Further, since such a light emitting element is a carrier-injection type light emitting element, driving at DC voltage is possible and noise is not easily generated.
In addition to element characteristics such as thinness, lightweight, high-speed response, and direct-current low-voltage driving as described above, it can be also said to be one of great advantages that a color of light emission from a light emitting element using an organic compound is rich in variation. It is because of the variety of organic compounds themselves. That is, richness of colors is produced by flexibility of the organic compound in which materials having various colors of light emission can be developed by molecular design (for example, introduction of a substituent) or the like. It can be said that the biggest application field of a light emitting element utilizing this richness of colors is a full-color flat-panel display.
It can be said that the above-described element characteristics such as thinness, lightweight, high-speed response, and DC low-voltage driving are also appropriate characteristics for a flat-panel display. In recent years, the use of phosphorescent materials instead of fluorescent materials has been tried as an attempt at further improvement in light emitting efficiency. In a light emitting element using an organic compound, light emission from an excited singlet state (S*) (fluorescence) and light emission from an excited triplet state (T*) (phosphorescence) can be exhibited. When a fluorescent material is used, only light emission (fluorescence) from S* contributes.
However, it is considered that a statistical generation ratio of S* to T* of a light emitting element is S*:T*=1:3 (for example, see Non-Patent Document 1). Accordingly, in the case of a light emitting element using a fluorescent material, the theoretical limit of an internal quantum efficiency (the ratio of generated photons to injected carriers) is considered to be 25% on the ground of S*:T*=1:3. In other words, in the case of a light emitting element using a fluorescent material, at least 75% of injected carriers are wasted uselessly.
Conversely, it is believed that luminous efficiency is improved (simply, by 3 to 4 times) if light emission from T*, that is, phosphorescence can be used. However, in the case of a commonly used organic compound, light emission from T* (phosphorescence) is not observed at room temperature, and normally, only light emission from S* (fluorescence) is observed. In recent years, however, light emitting elements in which energy that is emitted while returning from T* to a ground state (hereinafter, referred to as triplet excitation energy) can be converted into light emission have been released one after another, and the high light emission efficiency thereof has attracted attentions (for example, see Non-Patent Document 2).
In Non-Patent Document 2, an iridium complex using a dibenzo[f,h]quinoxaline derivative for a ligand is synthesized and used as a light emitting substance of a light emitting element. The obtained light emitting element has high luminous efficiency; however, the color of light emission therefrom is orange-red, and red light emission with high color purity is not realized.
On the other hand, in Non-Patent Document 3, deep-red light emission with CIE chromaticity coordinates of (x, y)=(0.70, 0.28) is achieved with an iridium complex having 2,3-diphenylquinoxaline as a ligand.
[Non-Patent Document 1]
Tetsuo TSUTSUI, Textbook for the 3rd Workshop, Division of Molecular Electronics and Bioelectronics, Japan Society of Applied Physics, p. 31 (1993).
[Non-Patent Document 2]
J. Duan et al., Advanced Materials, (2003), 15, No. 3, Feb. 5, pp. 224-228
[Non-Patent Document 3]
Hiroyuki FUJII et al., IECE TRANS. ELECTRON., vol. E87-C, No. 12, December (2004), pp. 2119-2121
However, chromaticity coordinates of red are (x, y)=(0.67, 0.32), according to the NTSC (National Television System Committee) standard which is a standard for a full-color display. Therefore, when the iridium complex disclosed in Non-Patent Document 3 is used in a display device, chromaticity coordinates in a sending side from which image information is sent and those in a receiving side are not identical, accordingly, color reproducibility is not favorable. In addition, a wavelength obtained from a light emitting element is 675 nm, which means that spectral luminous efficiency is low compared with standard red; therefore, high luminance can not be obtained.